This invention relates to the art of vacuum brazing of metals, and more particularly to a new and improved brazing oven and method of operating the same.
When brazing aluminum or other materials in production operations, the brazing is performed under vacuum conditions to prevent the formation of metal oxides which would impede the formation of smooth, sound brazed joints. For example, in a typical aluminum brazing process, the materials to be joined by braze are moved into a vacuum oven. The air is evacuated from the oven, and heat is applied to the materials, typically by induction or radiant heaters, to cause the brazing to occur. After brazing, the oven is refilled with air to equalize oven and atmospheric pressure so that the oven door may be opened to allow the materials to be removed.
A difficult problem which is often encountered in this process lies with the introduction of unconditioned, i.e. water ladened, air into the oven during the exchange of materials to and from the oven. The introduction of water into the oven is troublesome because during the oven pump down, i.e. air evacuation, process before brazing, any water present will turn into a gas form under heat and reduced pressure conditions and add gas, i.e. water vapor, into the oven, thus requiring increased pump down time and increased production time. Also, more importantly, in aluminum brazing operations, magnesium and other materials are released from the brazing materials or brazing fluxes, and are deposited on the walls of the oven. The magnesium, or other material, being highly hydroscopic will readily absorb moisture, i.e. water, particularly at high temperature, from the air to form hydrous magnesium. Before a deep vacuum can again be reached in the oven, the absorbed water must leech from the hydrous magnesium which requires a variable pump down time, depending upon the amount of absorbed water, thus increasing dramatically the production time and/or the variation in time to complete a brazing cycle.
The typical solution to this problem has been to refill the oven only with conditioned, i.e. dry, air after the braze cycle and before oven door opening. However, this is only a partial solution because upon door opening, the dry refilling air which has been heated by the heated materials and oven walls, being less dense and lighter than the cooler atmospheric air outside the oven, will flow through the upper section or area of the open door and will be replaced with unconditioned air, i.e. moisture ladened, from the atmosphere through the lower section or area of the door, thus introducing water into the next pump down and brazing cycle. Other prior art solutions have involved adding pumps or pump capacity in view of the increased pump downtime caused by moisture in the air.